The present invention relates generally to trigger holdoff systems for oscilloscopes, and particularly to a trigger holdoff system for controlling the inhibit/enable cycle of the acquisition system of a digital oscilloscope.
In a digital storage oscilloscope, an input waveform is sampled at equally spaced time points along the time axis of the waveform, converted to digital data for storage, and displayed on a screen as an amplitude-versus-time waveform. That is, the input signal is sampled in response to a sampling clock and converted to digital data which is stored in a so-called acquisition memory under the control of a time-base control and address generator circuit. the input signal is also applied to a trigger generator which generates a trigger signal to be applied to the time-base control circuit. The time-base control circuit is enabled under control of a microprocessor and, when triggered, initiates an acquisition cycle to fill an acquisition memory.
After completion of the acquisition cycle, the acquired waveform is transferred to a display memory under control of the microprocessor. the microprocessor suitably may also process the waveform data for producing interpolation data, jitter correction and perhaps other waveform refinements, with the processed result being sent to the display memory. When such processes are completed, the microprocessor enables the time-base control circuit again to allow it to respond to another trigger from the trigger generator. However the acquisition cycle may not begin at the identical trigger point on the input signal because the processing time of the microprocessor varies, depending upon such factors as the kind of process and the number of processes. Therefore, if repetitious acquisition-and-display cycles are employed, the waveform displayed on the screen is unstable. This is particularly noticeable at the higher sweep speeds.